3d-beam modulation filter for equalizing dose and image quality in breast ct

ABSTRACT

A size and/or shape specific 3D-beam modulation filter and a size and/or shape specific immobilizer are provided for cone-beam breast computed tomography (bCT). The immobilizer places the breast on an optimal position in the field of view of the scanner system and the 3D-beam modulation filter modulates the incident x-ray beam in the cone-angle (i.e. z-axis of the detector panel) and fan angle (i.e. x-axis of the detector panel) directions in order to improve equalization of the photon fluence incident upon the detector panel and reduce unnecessary radiation dose that the breast receives. Both the immobilizer and the 3D-beam modulation filter are selected among a plurality of alternatives based on the specific shape, size and/or shape or size of the person&#39;s breast.

RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.15/779,342 filed May 25, 2018, which is a US National Phase ApplicationUnder 371 of PCT/US2016/063701 filed Nov. 23, 2016, which claims benefitto prior-filed provisional Application No. 62/260,169 filed on Nov. 25,2015 entitled “3D-Beam Modulation Filter for Equalizing Dose and ImageQuality in Breast CT”, the contents of each of which is incorporatedherein in its entirety.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with Government support under Grant No. EB002138awarded by the National Institutes of Health. The Government has certainrights in this invention.

BACKGROUND OF THE INVENTION

Conventional computed tomography (CT) systems for breast imaging do notaccount for the actual shape or size of the breast of the person. Mostconventional systems emit an approximately uniform distribution of x-rayphotons on the imaged object (e.g. the breast). As a result, theintensity of the x-ray beam that strikes the x-ray detector system ismuch lower under the thicker posterior part of the breast (closer to thechest wall) relative to the thinner anterior part of the breast, nearthe nipple. As a result, the intensity of the detected x-ray beam isinhomogeneous and the radiation dose is higher anteriorly relative tothe posterior portion of the breast.

Most conventional multi-detector CT systems (and more recently wholebody cone-beam CT systems) commonly utilize a bowtie-shaped filter tomodulate the incident x-ray fluence along the fan angle direction of adetector panel. Bowtie-shaped filters may improve the performance ofthese systems by reducing the required detector dynamic range, reducingscatter from the edge of the person, reducing person dose, and reducingthe effects of beam-hardening.

A conventional whole body CT system may include three separatebowtie-shaped filters to produce the same spectral shape and intensityat the detector using (1) a single material, (2) two differentmaterials, and (3) to reduce the beam hardening effect in thereconstructed image by adjusting the bowtie-shaped filter thickness sothat the effective attenuation for every ray is approximately the same.One dimensional bowtie filters have also been proposed for breast CTsystems, assuming the breast is a circular cylinder. However, thereduction in radiation dose provided by these conventionalbowtie-filters may be greatly improved if the actual breast shape of theperson being imaged is considered when designing the filter.

Embodiments of the present invention solve these problems and otherproblems, individually and collectively.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a 3D-beam modulation filter for dedicatedcone-beam breast computed tomography (bCT). The filter modulates theincident x-ray beam in the cone-angle (i.e. z-axis of the detectorpanel) and fan angle (i.e. x-axis of the detector panel) directions inorder to partially equalize the photon fluence incident upon thedetector panel and to reduce the radiation levels that some parts of thebreast receive. The filter attenuates x-ray intensity along the x-raypaths that are attenuated only slightly by the breast (i.e. anterior andperipheral regions of the breast), while attenuating much less of thex-ray intensity along the highly attenuating x-ray paths (i.e. theposterior and central regions of the breast). Use of the filter reducesradiation dose to the person and improves the image quality compared toconventional cone beam bCT systems, which do not use x-ray modulation inboth the cone angle and fan angle directions.

Embodiments provide a mechanism that may select a 3D-beam modulationfilter among a plurality of 3D-beam modulation filters based on theshape of the breast of a person. For example, embodiments may providefive different 3D-beam modulation filters designed based on actualbreast geometries of a large population average over a range of women(e.g. about 215 women) with different breast shapes and sizes. For agiven person, an image (e.g. an x-ray image or an optical image) of thebreast may be generated to determine the actual shape of the person'sbreast. One of the five different 3D-beam modulation filters may bedynamically selected based on the actual breast shape using motorizedwheels or translating systems. The 3D-beam modulation filter is used inbCT imaging to reduce the radiation dose received by the person and toimprove the image quality of the bCT.

In some embodiments, a method for acquiring a CT image of a body partusing a scanner system is provided. The method includes selecting animmobilizer among a plurality of immobilizers based on a shape or sizeof the body part. The method also includes selecting a 3D-beammodulation filter among a plurality of 3D-beam modulation filters basedon the shape or size of the body part. The 3D-beam modulation filter isplaced at a predetermined distance from an x-ray source of the scannersystem. The selected immobilizer is coupled to the system. In someembodiments, the selected immobilizer may be coupled to the scannersystem by attaching a first end of an attachment element to a surface ofthe scanner system, and attaching second end of the attachment elementto the selected immobilizer. The attachment element may include a flangeor a fastener. The method further includes positioning the body part inthe selected immobilizer and acquiring a computed tomography (CT) imageof the body part using the scanner system including the 3D-beammodulation filter. In some embodiments, acquiring the CT image of thebody part further comprises collecting x-rays beams emitted from thex-ray source of the scanner system on a detector panel of the scannersystem, wherein the x-ray beams emitted by the x-ray source are filteredby the 3D-beam modulation filter prior to traveling through the bodypart.

In some embodiments, the body part may be a breast. The 3D-beammodulation filter is designed to reduce unnecessary radiation dosetowards the anterior and peripheral regions of the breast. According tovarious embodiments, the method may also include identifying apredetermined profile among a plurality of predetermined profiles basedon the shape or size of the body part, wherein each of the plurality of3D-beam modulation filters and each of the immobilizers are generatedfor one of the plurality of predetermined profiles. The method may alsoinclude forming a plurality of molds corresponding to the plurality ofpredetermined profiles, and producing the plurality of immobilizersusing the plurality of molds. Producing the molds may also includeproducing a first immobilizer using a first mold corresponding to afirst predetermined profile, and producing a second immobilizer using asecond mold corresponding to a second predetermined profile. Embodimentsallow for dynamic adjustment of the 3D-beam modulation filter prior toacquiring the computed tomography (CT) image of the body part.

According to various embodiments, the 3D-beam modulation filter may be acombined filter. In such embodiments, the method may also includeselecting a bowtie-shaped filter among a plurality of bowtie-shapedfilters based on the shape or size of the body part, selecting awedge-shaped filter among a plurality of wedge-shaped filter based onthe shape or size of the body part, and combining the selectedbowtie-shaped filter and the selected wedge-shaped filter into thecombined filter.

Embodiments may further provide a computing device including anon-transitory storage medium storing instructions, and a processorexecuting the instructions stored on the non-transitory storage mediumto perform the method of described above.

Embodiments may also provide a computed tomography (CT) scanner systemincluding an x-ray production system including an x-ray source emittingx-ray beams and an x-ray detector system for receiving the x-ray beamsemitted by the x-ray source. The CT scanner system may also include a3D-beam modulation filter positioned between the x-ray source and thedetector system at a predetermined distance from the x-ray source. The3D-beam modulation filter is specific to a predetermined body part shapeor size. The CT scanner system may further include a gantry assemblysystem including a surface for receiving a body part to be imaged, andan immobilizer coupled to the gantry assembly system using one or moreattachment elements. The body part being imaged may conform to thepredetermined body part shape or size and the immobilizer is specific tothe predetermined body part shape or size.

In some embodiments, the CT scanner system may include a scanner controlcomputer coupled to the x-ray production system and the gantry assemblysystem for sending control signals to the x-ray production system andthe gantry assembly system. The CT scanner system may also include afilter positioning system for selecting the 3D-beam modulation filteramong a plurality of 3D-beam modulation filters based on thepredetermined body part shape or size, and for positioning the 3D-beammodulation filter between the x-ray source and the detector system atthe predetermined distance from the x-ray source. In some embodiments,the 3D-beam modulation filter includes a combined filter. For suchembodiments, the filter positioning system is further configured toselect a bowtie-shaped filter among a plurality of bowtie-shaped filtersbased on the predetermined body part shape or size, select awedge-shaped filter among a plurality of wedge-shaped filter based onthe predetermined body part shape or size, and combine the selectedbowtie-shaped filter and the selected wedge-shaped filter into thecombined filter.

According to various embodiments, the CT scanner system may also includean image acquisition computer for receiving image data from the x-raydetector system, an image reconstruction computer for reconstructing theCT image of the body part based on the image data received from theimage acquisition computer, and a display for displaying thereconstructed CT image of the body part. The image reconstructioncomputer receives data from the scanner control computer and the imageacquisition computer, the data including one or more of x-ray beamintensity data, projection images of the body part being imaged, x-raybeam emission timing data or gantry assembly system positioning data.

These and other embodiments are described in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary system for cone-beam breast computedtomography (bCT) in accordance with embodiments of the invention.

FIG. 2A illustrates x-ray beam path length location for posterior andanterior regions of the breast in the cone angle direction in accordancewith embodiments of the invention.

FIG. 2B illustrates x-ray beam path length location for central andperipheral regions of the breast in the fan angle direction inaccordance with embodiments of the invention.

FIG. 3. illustrates an exemplary system 300 for acquiring bCT imagesusing a 3D-beam modulation filter according to various embodiments ofthe present invention.

FIG. 4A illustrates a histogram of breast volumes from a large cohort ofbCT volume data sets of a plurality of actual persons according tovarious embodiments of the present invention.

FIG. 4B illustrates average radius profiles for a plurality of subsetsof breast sizes determined based on the data illustrated in FIG. 4Aaccording to various embodiments of the present invention.

FIG. 5 illustrates exemplary breast phantoms for breast class V1, breastclass V3 and breast class V5 (identified in FIGS. 4B-4C) according tovarious embodiments of the present invention.

FIG. 6 illustrates a workflow for fabricating breast immobilizers usingthe breast phantoms as molds in accordance with embodiments of theinvention.

FIG. 7A illustrates a schematic of the breast immobilization deviceincorporated on a tabletop of a bCT system in accordance withembodiments of the invention.

FIG. 7B illustrates a top view of the breast immobilization deviceillustrated in FIG. 7A in accordance with embodiments of the invention.

FIG. 8 illustrates a projection image of the breast phantomcorresponding to the breast class V3 acquired on a bCT scanner using thebreast immobilization device illustrated in FIGS. 7A-7B in accordancewith embodiments of the invention.

FIG. 9A illustrates a sequence of six projection images acquired usingfilters made of a selected filtration material (e.g. titanium) withincreasing thickness in accordance with embodiments of the invention.

FIG. 9B illustrates plots showing the added filtration as a function ofarbitrary detector unit (ADU) for two selected detector elements“dexels” of the detector panel within the entire sequence of projectionimages illustrated in FIG. 9A in accordance with embodiments of theinvention.

FIG. 10 illustrates an exemplary 3D-beam modulation filter thickness mapresulting from repeating the process shown in FIGS. 9A-9B for all dexelsof the detector panel in accordance with embodiments of the invention.

FIG. 11 illustrates an exemplary CAD model of the 3D-beam modulationfilter design in accordance with embodiments of the invention.

FIG. 12A illustrates Monte Carlo simulation results for a projection ofthe V3 phantom on the detector panel when the 3D-beam modulation filteris not used in accordance with embodiments of the invention.

FIG. 12B illustrates Monte Carlo simulation results for a projection ofthe V3 phantom on the detector panel when the 3D-beam modulation filteris used in accordance with embodiments of the invention.

FIG. 13A illustrates comparison of a first line profile (without using afilter) and a second line profile (using a filter) through the simulatedprojection images along the horizontal dimension shown in FIG. 12A.

FIG. 13B illustrates comparison of a first line profile (without using afilter) and a second line profile (using a filter) through the simulatedprojection images along the vertical dimension shown in FIG. 12A.

FIG. 14A illustrates Monte Carlo simulation estimations of thescatter-to-primary ratio (SPR) without using a 3D-beam modulation filterin accordance with embodiments of the invention.

FIG. 14B illustrates Monte Carlo simulation estimations of thescatter-to-primary ratio (SPR) using a 3D-beam modulation filter inaccordance with embodiments of the invention.

FIG. 15A illustrates comparison of a first line profile (without using afilter) and a second line profile (using a filter) through the simulatedSPR maps images along the horizontal dimension shown in FIG. 14A.

FIG. 15B illustrates comparison of a first line profile (without using afilter) and a second line profile (using a filter) through the simulatedSPR maps along the vertical dimension shown in FIG. 14A.

FIG. 16 illustrates cone angle path length profile results combining abreast CT images of a plurality of persons in accordance withembodiments of the invention.

FIG. 17A illustrates the measured ADU (near the location of the centralray on the detection plane) as a function of tube current in accordancewith embodiments of the invention.

FIG. 17B illustrates the measured ADU (near the location of the centralray on the detection plane) as a function of exposure at the isocenterfor the bCT scanner in accordance with embodiments of the invention.

FIG. 18A illustrates the computed filter thickness for an exemplarycopper wedge-shaped filter as a function of cone angle (data points) anda linear regression fit (solid line) in accordance with embodiments ofthe invention.

FIG. 18B illustrates the computed filter thickness for an exemplarytitanium wedge-shaped filter as a function of cone angle (data points)and a linear regression fit (solid line) in accordance with embodimentsof the invention.

FIG. 18C illustrates the computed filter thickness for an exemplaryaluminum wedge-shaped filter as a function of cone angle (data points)and a linear regression fit (solid line) in accordance with embodimentsof the invention.

FIG. 19A illustrates the thickness profile in the z-direction of anexemplary aluminum wedge-shaped filter for each breast class inaccordance with embodiments of the invention.

FIG. 19B illustrates the thickness profile in the z-direction of anexemplary copper wedge-shaped filter for each breast class in accordancewith embodiments of the invention.

FIG. 19C illustrates the thickness profile in the z-direction of anexemplary titanium wedge-shaped filter for each breast class inaccordance with embodiments of the invention.

FIG. 20 illustrates fan angle path length profile results combiningbreast CT images from a plurality of persons in accordance withembodiments of the invention.

FIG. 21A illustrates the computed filter thickness for an exemplarycopper bowtie-shaped filter as a function of cone angle (data points)and a linear regression fit (solid line) in accordance with embodimentsof the invention.

FIG. 21B illustrates the computed filter thickness for an exemplarytitanium bowtie-shaped filter as a function of cone angle (data points)and a linear regression fit (solid line) in accordance with embodimentsof the invention.

FIG. 21C illustrates the computed filter thickness for an exemplaryaluminum bowtie-shaped filter as a function of cone angle (data points)and a linear regression fit (solid line) in accordance with embodimentsof the invention.

FIG. 22A illustrates the thickness profile in the x-direction of anexemplary aluminum bowtie-shaped filter for each breast class inaccordance with embodiments of the invention.

FIG. 22B illustrates the thickness profile in the x-direction of anexemplary copper bowtie-shaped filter for each breast class inaccordance with embodiments of the invention.

FIG. 22C illustrates the thickness profile in the x-direction of anexemplary titanium bowtie-shaped filter for each breast class inaccordance with embodiments of the invention.

FIG. 23 is a diagram of combining an exemplary bowtie-shaped filter andan exemplary wedge-shaped filter used in tandem in accordance withembodiments of the invention.

FIG. 24A illustrates the 3D shape of an aluminum combined bowtie-shapedand wedge-shaped filter using a surface plot of the aluminum combinedfilter thickness profile in the z-direction and x-direction inaccordance with embodiments of the invention.

FIG. 24B illustrates characteristics of an aluminum combinedbowtie-shaped and wedge-shaped filter using lines profiles at variousz-locations in accordance with embodiments of the invention.

FIG. 24C illustrates characteristics of an aluminum combinedbowtie-shaped and wedge-shaped filter using lines profiles at variousx-locations in accordance with embodiments of the invention.

FIG. 25 illustrates the combined filter shape of titanium 3D-beammodulation filter for the first profile (e.g. V1, the x-small sizebreast) and the combined filter shape of titanium 3D-beam modulationfilter for the fifth profile (e.g. V5, the x-large size breast).

FIG. 26 illustrates a flowchart of steps for generating a CT image of abody part using a selected 3D-beam modulation filter in accordance withembodiments of the invention.

FIG. 27 illustrates shows an exemplary computer system, in accordancewith embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a 3D-beam modulation filter that improvesthe image quality and reduces the x-ray exposure (e.g. radiation dose)on the breast of a person. The 3D-beam modulation filter is designedbased on the specific breast shape or size of a given person. The3D-beam modulation filter is used in breast CT (bCT) imaging to reducethe radiation dose received by the person during the examination and toimprove the image quality of the bCT. The invention described hereinuses a large cohort of person bCT volume datasets to design the 3D-beammodulation filters and, as such, is based on a relatively largepopulation average over a range of women with different breast shapesand sizes. The present methodology for designing a 3D-beam modulationfilter is derived from projection images measured on a bCT system,making it well suited for flexibility across many different imaginggeometries, x-ray techniques, and objects being imaged. According tovarious embodiments, the systems and methods described herein are notlimited for breast imaging but may be used in connection with variousparts of a human or animal body. In addition, the 3D-beam modulationfilters may be designed for many different breast sizes and a filterpositioning device may allow for the selection and adjustment of thefilter on a person-specific basis.

Overview of the Exemplary System

FIG. 1 illustrates an exemplary system 100 for cone-beam breast computedtomography (bCT) in accordance with embodiments of the invention. Asillustrated in FIG. 1, a person 102 may be positioned on a table 104 inthe prone position with the breast 106 to be imaged hanging pendantbetween an x-ray source 108 and a flat panel detector 110. The x-raysource 108 and the flat panel detector 110 are coupled to a gantry 112that rotates around the person's body part (e.g. the breast 106) to scanthe body part beneath the table 104. In some embodiments, the breast maybe provided in a breast immobilizer (e.g. a breast immobilizing cone ora breast immobilization device) that positions the breast at the centerof the field of view (FOV) 114. The x-ray source 108 and the flat paneldetector 110 are connected to the gantry 112 that rotates around theperson's breast 106 (provided in the breast immobilizer). The rotationalplane of the gantry 112 is the plane of the fan angle (i.e. x-axis ofthe detector panel 110 as illustrated in FIG. 1).

The signal received at the detector panel 110 is dependent on the pathlength travelled by the photons emitted by the x-ray source 108 as theypass through the breast tissue within the detector field of view (FOV)114. Large differences in breast thickness as a function of locationalong the detector panel 110 results in unequal photon fluence strikingthe detector 110. Given that the relative image noise is inverselyproportional to the number of detected photons, the noise in the coneangle direction (i.e., z-axis of the detector panel 110 as illustratedin FIG. 1, the vertical direction of the detector panel 110) of acone-beam projection of the breast increases towards the posteriorregion 204 of the breast 106 due to its greater x-ray attenuation inthis relatively thick region as shown in FIG. 2A. Specifically, FIG. 2Aillustrates x-ray beam path length location for posterior 204 andanterior 206 regions of the breast 106 in the cone angle direction inaccordance with embodiments of the invention.

Similarly, the noise in the fan angle direction (i.e., y-axis of thedetector panel 110 perpendicular to x-axis and z-axis illustrated inFIG. 1, the horizontal direction of the detector panel 110) of acone-beam projection of the breast increases towards the central region214 of the breast 106 due to its greater x-ray attenuation in thisrelatively thick region as shown in FIG. 2B. Specifically, FIG. 2Billustrates x-ray beam path length location for central 214 andperipheral 216 regions of the breast 106 in the fan angle direction inaccordance with embodiments of the invention.

The highest noise levels are typically seen in the thicker central andposterior parts of the bCT images and lower noise levels are typicallyseen on the periphery and anterior parts of the bCT images. This is dueto the reconstruction process used to compute the bCT images inherentlypropagating the noise.

Radiation dose coefficients in bCT are greater for smaller diameterbreasts relative to larger diameter breasts based on Monte Carlosimulations of cylindrical phantoms. In reality, the breast 106 inpendant geometry (as illustrated in FIG. 1) is more of a conical shapethat tapers in diameter towards the nipple. Given a relatively uniformx-ray spectrum incident upon the breast 106, a conical shaped breastwill receive a slightly higher radiation dose anteriorly (e.g. at andaround the nipple) and lower radiation dose posteriorly (e.g. closer tothe chest wall). Thus, a 3D-beam modulation filter 116 is designed inthe present invention to reduce the unnecessary dose towards theanterior and peripheral regions of the breast, respectively, where thehigher detected signal (i.e. lower noise) does not substantiallycontribute to improved image quality. As illustrated in FIG. 1, the3D-beam modulation filter 116 may be positioned between the x-ray source108 and the breast 106, at a predetermined distance from the x-raysource 108. According to an exemplary embodiment, the predetermineddistance between the 3D-beam modulation filter 116 and the x-ray source108 may be about 10 cm. However, the predetermined distance is notlimited to the exemplary embodiment and can be any suitable distancethat allows for radiation dose reduction on the person withoutcompromising on the image quality.

FIG. 3 illustrates an exemplary system 300 for acquiring bCT imagesusing a 3D-beam modulation filter according to various embodiments ofthe present invention. The system 300 includes imaging and computingmodules as well as the components of the exemplary system 100 forcone-beam breast computed tomography (bCT) illustrated in FIG. 1.Specifically, the system 300 includes an x-ray production system (e.g.the x-ray source 108), a gantry assembly system 304 (e.g. the gantry 112and the table 104), an x-ray detector system 306 (e.g. the detectorpanel 110), and a scanner control computer 308. The scanner computer 308may send control signals to the x-ray production system 302 to adjustthe intensity and/or timing of the x-ray beams produced by the x-rayproduction system 302. The scanner computer 308 may also send controlsignals to the gantry assembly 304 to correctly position the personrelative to the x-ray production system 302. When the desiredconfiguration is achieved, the scanner control computer 308 may send acontrol signal to the x-ray production system 302 to start the x-raybeam production by the x-ray production system 302.

According to various embodiments, the system 300 may also include afilter positioning system 316. The filter positioning system 316 maychange the 3D-beam modulation filter to be used during the scanningprocess based on the breast class of the user. As further describedbelow, embodiments of the present invention are directed to forming adedicated 3D-beam modulation filter for each identified breast class.Accordingly, a first 3D-beam modulation filter may be used for personshaving a first breast class and a second 3D-beam modulation filter forpersons having a second breast class. The scanner computer system 308may receive a determination of the person's breast class (e.g. throughuser input or through input from an evaluating system that evaluatesperson's breast to determine the breast profile) and send a signal tothe filter positioning system 316 to select the 3D-beam modulationfilter corresponding the person's breast class among the plurality of3D-beam modulation filters, and place the selected 3D-beam modulationfilter at a predetermined distance from the x-ray production system 302,between the x-ray production system 302 and x-ray detector system 306.

According to various embodiments, the scanner computer system 308 maysend signals to the filter positioning system 316 and the gantryassembly system 304 to adjust the respective positions of the 3D-beammodulation filter and the gantry if the person's breast is not at anoptimal location within the field of view of the laser beam. Forexample, the position of the 3D-beam modulation filter and the gantrymay be adjusted according to a first person with a first breast class.When a second person is placed on the gantry, adjustment to theplacement of the filter and/or the gantry may be necessary. If thesecond person has a second breast class, then the scanner controlcomputer 308 may instruct the filter positioning system 316 to changethe filter to the 3D-beam modulation filter corresponding the secondbreast class. On the other hand, even if the second person has the firstbreast class (e.g. the same breast class as the previous person), thesecond person may be lighter/heavier than the previous person thus mayresult in the table being pressed higher/lower than the previous person.This may result in the second person's breast not being optimallylocated within the field of view of the laser beam. In such cases, thescanner control computer 308 may instruct the gantry assembly system 304to go lower/higher to place the breast within the field of view of thelaser beam.

According to various embodiments, the system 300 may also include animage acquisition computer 310, an image reconstruction computer 312,and an image display software 314. After the x-ray production system 302starts the x-ray beam production, the scanner control computer 308 maynotify the image acquisition computer 310 to acquire one or more x-rayimages of the person's body part (e.g. breast). The image acquisitioncomputer 310 may receive or acquire (e.g. pull) the image data from thex-ray detector system 306. The image acquisition computer 310 may sendthe acquired image data to the image reconstruction computer 312. Theimage reconstruction computer 312 may also receive data (e.g. x-ray beamintensity data, x-ray beam emission timing data, the gantry assemblysystem positioning data, etc.) from the scanner control computer 308.The image reconstruction computer 312 may reconstruct the CT image ofthe person's breast based on the image data received from the imageacquisition computer 310 and display the reconstructed image using animage display software 314 on a display device. According to variousembodiments, the image display software 314 may be provided on the imagereconstruction computer 312, on a different computer or on a remoteserver (e.g. cloud storage).

The 3D-beam modulation filter 116 may compensate for the differences inthickness of the breast 106 and equalize the attenuation of the signallevels at the detector 110. However, for the 3D-beam modulation filter116 to work properly, the exact location of the breast 106 with respectto the x-ray source 108 and the detector 110 needs to be determinedand/or known. The breast 106 may be placed at a predetermined (e.g.known) position by using a breast immobilizer, discussed below. A seriesof breast-immobilizing molds are produced from a large cohort of breastCT volume datasets of women with different breast shapes and sizes. Thebreast immobilizers help to conform the breast to be imaged to becentered in the field of view, and positioned to optimally exploit theshape of the 3D-beam modulation filter.

The purpose of the breast immobilizer is to gently force the pendantbreast to conform to the shape of the breast immobilizer. The breastimmobilizer corresponding to the person's breast size/category may bepositioned over the hole on the table where the breast to be imaged isplaced. The 3D-beam modulation filter works together with thesize-specific breast immobilizer to equalize the signal levels at thedetector and reduce radiation dose in the anterior region and peripheryof the breast.

The breast immobilizers are generated using a plurality of realisticbreast-shaped phantoms that are pre-defined shapes formed using thelarge cohort of breast CT volume datasets of women with different breastshapes and sizes. For a given person, the breast is assumed to conformto one of the pre-defined breast-shaped phantoms. The breast-shapedphantoms are used as molds for creating the breast immobilizers.

Identification of Breast Classes

FIG. 4A shows a histogram 400 of breast volumes from a large cohort ofbreast CT volume data sets of a plurality of (e.g. 215) actual persons.One of ordinary skill in the art will appreciate that the number ofexperiment subjects may be modified as long as a statisticallysignificant group is analyzed.

As used herein, the breast CT volume data set may include a complete 3Dreconstruction of the object (e.g. body part, breast) using a pluralityof projection images (e.g. about 500 projection images) from a pluralityof x-ray source and detector panel positions (e.g. about 500 positions).

As used herein, a projection image may include a single image of anobject (e.g. body part) for a single x-ray source and detector panelposition.

According to exemplary embodiments, the total breast volume of 215dedicated breast CT volume data sets may be classified into a pluralityof percentile groups, e.g. 0-20^(th), 20-40^(th) 40-60^(th), 60-80^(th),and 80-100^(th) percentiles, corresponding to breast volumes, e.g.x-small (V1), small (V2), medium (V3), large (V4), and x-large (V5)breast volumes. Thus, the gathered data may be classified into five mainclasses based on the identified five percentile groups. According tovarious embodiments, the data may be classified into multiple classesbased on volume, size, shape or any other determining factor. In theexemplary embodiment illustrated in FIG. 4A, the data is classified byvolume. A volume index (e.g. V1, V2, V3, V4 and V5) may be assigned toeach identified breast class, each breast class corresponding to theaverage effective diameter profile within each of the five percentilegroup.

According to various embodiments, average effective radius profiles maybe measured within each of the breast classes V1-V5. The radius profilemeasurements may begin at the 1^(st) coronal slice containing no chestwall artifacts and end at the last coronal slice containing the person'snipple. FIG. 4B illustrates average radius profiles 402, 404, 406, 408,410 for a plurality of subsets of breast sizes (i.e. each of the breastclasses V1-V5) determined based on the data illustrated in FIG. 4A. Theaverage radius profiles 402, 404, 406, 408, 410 may form the basis forgenerating a breast phantom for each of the identified breast classes.For example, the average radius profile 402 corresponds to the breastclass V1 for x-small breast size, the average radius profile 404corresponds to the breast class V2 for small breast size, the averageradius profile 406 corresponds to the breast class V3 for medium breastsize, the average radius profile 408 corresponds to the breast class V4for large breast size, and the average radius profile 410 corresponds tothe breast class V5 for x-large breast size.

Each breast class may be associated with specific characteristics suchas chest wall diameter, breast length, breast volume, etc. The value fora given characteristic may correspond to average value of thatcharacteristic for each member in the breast class. Table 1 illustratesthe average chest wall diameter, the average length, the average volumeand the average volumetric growth factor (VGF) for each of thepercentile groups corresponding to the breast classes identified byvolume index V1-V5. One of ordinary skill in the art will appreciatethat the data may be categorized into any number of groups and based onany relevant criteria. The use of 5 groups based on percentile groupingis provided for illustrative purposes only and should not be construedas limiting.

TABLE 1 Properties of identified breast volumes Size Chest Wall LengthIndex Diameter (mm) (mm) Volume (ml) VGF (%) V1 108.2 54 251.4[26.0-395.0)   22.2 V2 121.6 80 499.4 [395.0-586.5)  16.4 V3 132.1 89680.4 [586.5-802.1)  13.2 V4 138.6 99 928.2 [802.1-1078.8) 10.5 V5 149.2106  1505.3 [1078.8-2450.5) 13.5

One of ordinary skill in the art will appreciate that the groupingprovided herein is for illustrative purposes and that the data may beclassified into more or less volume-classified groups. Moreover, theeffective diameter profiles may be classified by other anatomicalmetrics besides volume (i.e. breast diameter). A breast immobilizer(e.g. breast immobilizing mold) and a 3D-beam modulation filter of agiven material (e.g. aluminum, copper, titanium) may be generated foreach breast class.

Breast Phantoms and Breast Immobilizers

The effective radius profiles 402, 404, 406, 408, 410 may be used tofabricate breast phantoms, e.g. polyethylene breast phantoms, for breastclasses V1, V2, V3, V4 and V5, that represent the realistic breastvolume and shape for each of the breast classes. Exemplary polyethylenephantoms for breast class V1 500, breast class V3 502 and breast classV5 504 are illustrated in FIG. 5. The phantoms illustrated in FIG. 5 arenot drawn to scale. One of ordinary skill in the art would appreciatethat the phantom for breast class V5 504 (i.e. phantom for x-largevolume) is larger than both the phantom for breast class V1 500 (i.e.phantom for x-small volume) and the phantom for breast class V3 502(i.e. phantom for medium volume). Similarly, the phantom for breastclass V3 502 (i.e. phantom for medium volume) is larger than the phantomfor breast class V1 500 (i.e. phantom for x-small volume).

As provided above, the phantoms may then be used to form the breastimmobilizers. For example, a breast immobilizer corresponding to eachone of the identified breast classes may be formed. FIG. 6 illustrates aworkflow 600 for fabricating breast immobilizers using the breastphantoms as molds. At a first step, a sheet of moldable material (e.g. athermoplastic sheet) 602 may be placed in a hot molding environment(e.g. hot water bath) 604 to be softened. The softened sheet of moldablematerial may be placed over one of the phantoms for breast classes V1-V5for the molding process 606. The softened sheet of moldable material maytake the shape of the phantom. When set (e.g. by being cured or anyother method), the molded sheet of moldable material may form the breastimmobilizer 608 corresponding to the phantom/breast class. According tovarious embodiments, a breast immobilizer may be formed for each breastclass phantom (e.g. V1-V5). The breast immobilizer may optimize theeffect of the 3D-beam modulation filter by containing the person'sbreast to a pre-determined shape and location.

FIG. 7A illustrates a schematic of the breast immobilization device 700incorporated on the tabletop 702 of a bCT system. The immobilizationdevice 700 may include attachment elements 704 and a breast immobilizer708 for placing the person's chest into the x-ray source field of view(FOV) to maximize chest wall coverage. A selected thermoplasticimmobilizer (e.g. breast immobilizer) 708 may be attached to thetabletop 702 of the bCT system using one or more attachment elements 704(e.g. a flange, a neoprene flange, a fastener, a hook-and-loop fastener,etc.). According to some embodiments, the thermoplastic immobilizer 708may perforated. When the person's breast is placed in the thermoplasticimmobilizer 708, the perforation holes may allow for a visual inspectionof the goodness of fit between the breast and the thermoplasticimmobilizer 708. For example, if the breast protrudes through the holes,a larger size thermoplastic immobilizer 708 may be required. Similarly,if there is a gap between the surface of the thermoplastic immobilizer708 and the outer surface of the person's breast, a smaller sizethermoplastic immobilizer 708 may be required. According to variousembodiments, the check for the immobilizer fit may be performed by alaser-based system which may scan the immobilizer to detect protrudingskin or a void just beneath the immobilizer surface which can then becompared to a threshold. The system may provide a recommendation tochange the immobilizer based on the input from the laser detectionsystem. Alternatively, the check for the immobilizer fit may beperformed by a person, such as the x-ray technician.

According to various embodiments, several different attachment elements704 with different mechanical properties may be employed depending onthe person's body habitus, for the purpose of both suspending theperson's breast and chest wall into the scanner field of view (FOV) andsimultaneously holding back tissue that is not of interest for the bCTacquisition. In some embodiments, the attachment element 704 may bepermanently attached to the tabletop 702 of the bCT system. The breastimmobilizer 708 may be coupled to the tabletop 702 by way of theattachment element 704. For example, a first end of the attachmentelement 704 may be attached to the tabletop 702 of the bCT system whilea second end, opposite from the first end, of the attachment element 704may be attached to the breast immobilizer 708. The thermoplastic breastimmobilizer 708 may be replaced depending on the volume and shape of theperson's breast by being detached from the attachment element 704 suchthat any one of the previously formed breast immobilizers (e.g. forprofiles V1-V5) may be attached to the tabletop 102 via the attachmentelement 704. In the exemplary embodiment illustrated in FIG. 7A, abreast phantom 706 may be placed in the breast immobilizer 708. FIG. 7Billustrates a top view of the breast immobilization device 700 includingthe attachment element 704, the breast immobilizer 708 and the phantom706 placed in the breast immobilizer 708.

Using the system setup illustrated in FIG. 7A, the breast phantoms maythen be used to design different 3D-beam modulation filters such thateach 3D-beam modulation filter perfectly matches the shape of eachbreast phantom, as discussed below.

Design of 3D-Beam Modulation Filter

The breast phantoms (e.g. phantoms for breast classes V1-V5) may be usedto design five size-specific 3D-beam modulation filters. The filterdesign for a specific bCT scanner may depend on the system geometry andthe x-ray technique of the scanner system. According to variousembodiments, projection images may be obtained on the bCT system beinganalyzed with the breast phantom placed at the scanner isocenter.

FIG. 8 illustrates a projection image 802 of the phantom (without thebreast immobilizer) corresponding to the breast class V3 at 65 kV withinherent filtration of 0.5 mm titanium filter acquired on a bCT scanner.The minimum detected signal (ADU_(min)) 804, measured in arbitrarydetector unit (ADU), may be used as a normalization factor for signalequalization. ADU_(min) value may be within the dynamic range of thedetector panel. In some embodiments, ADU_(min) value may be near theupper limit of the dynamic range of the detector panel such that thesignal is not quantum noise limited. As shown in FIG. 8, the dexellocation of ADU_(min) coincides with the thickest region of the breastat the posterior edge. If the entire projection image was perfectlyequalized, all detector elements (i.e. dexels) would be equal toADU_(min).

A plurality of projection images may be acquired with increasingthicknesses of filtration material until the ADU value in every dexel isequal to or less than ADU_(min). FIG. 9A illustrates a sequence of sixprojection images 900, 902, 904, 906, 908 and 910 acquired using filtersmade of a selected filtration material (e.g. Grade-5 titanium alloy)with thickness increasing from 0 mm to 3.6 mm, respectively.

The added filtration as a function of ADU may be plotted for each dexelwithin the entire sequence of projection images 900-910. Respectivecurves 920 and 922 corresponding to two exemplary dexel elements 930 and932 are shown in FIG. 9B. The circle and triangle markers correspond tothe individual dexel elements 930 and 932 illustrated in FIG. 9A.Similar curves may be plotted for each detector element (i.e. dexel) todetermine the thickness of the 3D-beam modulation filter for a specificdetector element. For example, if a region of interest is closer to thenipple of the breast, a first thickness may be selected for the filter.On the other hand, if the region of interest is closer to the chestwall, a second thickness (different than the first thickness) may beselected for the filter. The curves may be interpolated for the detectedsignal equal to ADU_(min) 930 to calculate the amount of filtration 932and 934 each ray needs to pass through as the ray originates from thefocal spot to reach the particular dexel of interest.

FIG. 10 shows an example of the 3D-beam modulation filter thickness map1000 resulting from repeating the process shown in FIGS. 9A-9B for alldexels within the detector panel. The thickest region of breast is atthe central-posterior edge 1002 of the breast near the chest wall. Thisregion requires less filtration than the region closer to the anterioredge 1004 of the breast near the nipple where the projection thicknessis much thinner.

FIG. 11 illustrates an exemplary CAD model 1100 of the 3D-beammodulation filter design according to various embodiments. Theimage-derived design for the 3D-beam modulation filter discussed hereinis a robust method in that the resulting 3D-beam modulation filterdesign inherently includes the effects of the particular scannergeometry, x-ray beam hardening, object scatter, and differences indetector spectral response. One of ordinary skill in the art willappreciate that the same design method may be applied to any breastphantom size and any filtration material of interest. The design methodcan also be applied to other anatomical object on a person forapplications outside of breast imaging.

According to various embodiments, a 3D-beam modulation filter may beused to generate an image that almost looks gray except for a finestructure (e.g. lesion of interest) detected in the woman's breast. Thatis, in some embodiments, an equalization filter may be designed tocompletely flatten the image and make it almost homogeneous exposure atthe detector. The results shown in FIG. 10 assumes that a perfectlyuniform detected signal is desired across the entire detector panel.However, this assumption may have adverse effects on the reconstructedimage quality, mainly beam hardening artifacts and contrast-to-noisedegradation. To address this issue, the 3D-beam modulation filterthickness map illustrated in FIG. 10 may be adjusted such that signalequalization is less aggressive.

Once the 3D-beam modulation filter is designed, a simulation techniquesuch as Monte Carlo simulation may be used to estimate the performanceof the resulting 3D-beam modulation filter. FIGS. 12A-12B illustrateMonte Carlo simulation results for projection images of the V3 phantom.FIG. 12A illustrates Monte Carlo simulation results 1200 for projectionimages of a V3 breast phantom on the detector panel without using a3D-beam modulation filter. FIG. 12B illustrates Monte Carlo simulationresults 1210 for projection images of a V3 breast phantom on thedetector panel using a 3D-beam modulation filter. The simulation results1200 and 1210 may be normalized to ADU_(min) and displayed using thesame color scale.

FIGS. 12A-12B illustrate the large reduction in dynamic rangerequirement of the detector with introduction of the 3D-beam modulationfilter. Specifically, FIG. 12A shows the image of the V3 phantomacquired without using a filter. In this configuration the dexels withinthe breast phantom silhouette need to have a high enough detected signalto not be dominated by quantum noise and therefore results in high noiselevels in the reconstructed images as discussed previously. The dexelelements outside of the breast silhouette simultaneously need to bebelow the saturation level for that detector panel. This configuration(e.g. the image illustrated in FIG. 12A) results in a large dynamicrange requirement for the flat panel detector. On the other hand, theimage illustrated in FIG. 12B requires a detector panel with much lessof a dynamic range requirement.

FIGS. 13A-13B illustrate comparison of line profiles through thesimulated projection images shown in FIGS. 12A-12B, respectively. FIG.13A illustrates comparison 1300 of a line profile 1302 (without using afilter) and a line profile 1304 (using a filter) through the simulatedprojection images along the horizontal dimension 1202 (i.e. z=2 cm fromthe chest wall) shown in FIG. 12A. FIG. 13B illustrates comparison 1310of a line profile 1302 (without using a filter) and a line profile 1304(using a filter) through the simulated projection images along thevertical dimension 1204 (i.e. midline of the breast) shown in FIG. 12A.FIGS. 13A-13B further validate a substantial flattening of the projectedprofile at the periphery (FIG. 13A) and anterior (FIG. 13B) region ofthe breast.

Another important metric in assessing the impact of the 3D-beammodulation filter is the amount of scatter in each projection imagewhich results in increased noise levels in the reconstructed bCT volumedata sets. The scatter-to-primary ratio (SPR) is the ADU resulting fromscatter (single or multiple events) divided by the ADU resulting fromonly primary incident radiation. FIGS. 14A-14B illustrate the MonteCarlo simulation estimations of the scatter-to-primary ratio (SPR)demonstrating an overall reduction in the SPR near the center of thebreast. FIG. 14A illustrates the Monte Carlo simulation estimations ofthe SPR 1400 with no 3D-beam modulation filter. FIG. 14B illustrates theMonte Carlo simulation estimations of the SPR 1410 with the 3D-beammodulation filter. The average SPR within a 1 cm×1 cm square region atthe posterior, mid-line region of the projection image was reduced by31% after introducing the 3D-beam modulation filter.

FIGS. 15A-15B illustrate comparison of line profiles through thesimulated SPR maps. FIG. 15A illustrates comparison 1500 of a lineprofile 1502 (without using a filter) and a line profile 1504 (using afilter) through the simulated SPR maps along the horizontal dimension1402 (i.e. z=2 cm from the chest wall) shown in FIG. 14A. FIG. 15Billustrates comparison 1510 of a line profile 1502 (without using afilter) and a line profile 1504 (using a filter) through the simulatedSPR map along the vertical dimension 1404 (i.e. midline of the breast)shown in FIG. 14A. FIGS. 15A-15B further demonstrate the reduction inSPR near the posterior, mid-line region of the breast. As providedabove, designing a 3D-beam modulation filter that perfectly equalizesthe detected signal may not be an optimal choice and this is evidentgiven the increase in SPR near the periphery of the breast, asillustrated in FIG. 15A.

The dose deposited to the radiosensitive tissue within the breast (e.g.fibroglandular tissue) may also be estimated using Monte Carlosimulation techniques. For example the deposited dose estimation may bedetermined for the V3 phantom at a volumetric glandular fraction of 17%and a 1.5 mm thickness. The reduction in glandular dose afterintroducing the 3D-beam modulation filter was 34%, 45%, and 40% for theV1, V3, and V5 phantoms, respectively, when the dose was normalized tothe number of quanta reaching the detector under the thickest region ofthe breast. This normalization was used to compare the dose for anequivalent signal-to-noise ratio (SNR) at the detector. The resultsshown here demonstrate that at a constant SNR the introduction of a3D-beam modulation filter results in a large reduction in radiation dosedelivered to the person. It also demonstrates that at equivalent doselevels the configuration with a 3D-beam modulation filter would have ahigher SNR than the configuration without a 3D-beam modulation filter.

As provided above, the present invention uses a large cohort of personbCT volume datasets to design a 3D-beam modulation filter. According tosome embodiments, the 3D-beam modulation filter may include a combinedwedge and bowtie-shaped filter. A wedge-shaped filter is used to varythe intensity of the x-ray beam in the cone angle direction tocompensate for the differential thickness of breast tissue from theposterior 204 to the anterior 206 regions of the breast. A bowtie-shapedfilter is used to vary the intensity of the x-ray beam in the fan angledirection to compensate for the differential thickness of breast tissuefrom the central 214 to the peripheral 216 regions of the breast. Thedesign of the wedge-shaped filter, the bowtie-shaped filter and thecombined wedge and bowtie-shaped filter are discussed next.

Design of the Wedge-Shaped Filter

Using the specific geometrical specifications of the bCT scanner ofinterest (e.g. source to isocenter distance (SIC), source to detectordistance (SID), location of the central ray on the detector, etc.), aperson's breast image may be used to analytically determine x-ray pathlength as a function of position along the z-axis of the detector panel(i.e. cone-angle direction). Referring back to FIG. 2A, ray paths in thecone angle direction incident on the detector panel 110 are illustrated.By measuring the x-ray path length as a function of cone angle along themidline z-axis of the detector panel 110 for a single projection, acomplete description of the signal attenuation may be determined alongthe z dimension. This process may be repeated for any number ofprojections. Results of the combined path length profiles for 215 personimages (2 orthogonal views for each case) are shown in FIG. 16 usingimages acquired on a prototype bCT system. The curve 1600 illustrated inFIG. 16 represents the mean path length traveled by the x-rays throughthe breast from the chest wall to the nipple in the z-direction. Thewedge-shaped filter is designed to compensate for the large differencein path length 1600 illustrated in FIG. 16.

As it would be expected, the person's breast is thickest near the chestwall and tapers off towards the nipple. Depending on the availability ofperson data sets, any number of images may be combined to determine an“average” path length profile (e.g. the curve 1600 illustrated in FIG.16) for a given bCT system. The exemplary curve 1600 shown in FIG. 16takes the average path length profile for all person data sets (i.e.data sets for 215 persons) independent of breast size and shape.According to various embodiments, the path length profiles may beclassified by person diameter, volume, and/or any other geometricalconstraint resulting in different path length profiles for each personclass. For example, the path length profiles illustrated in FIG. 16 maybe grouped under 5 different classes such as x-small, small, medium,large and x-large, based on the breast size as discussed previously. Awedge-shaped filter may be formed for each category. Then, for a givenperson, one of the five wedge-shaped filters may be selected based onthe actual breast shape or sizeshape for the person.

The design considerations for the thickness of the wedge-shape filterare discussed next.

Using the exponential attenuation of photons, the photon fluence orexposure (using a photon fluence to exposure conversion factor) at thedetector may be determined for each path length within the entire pathlength profile 1600 illustrated in FIG. 16. In order to equalize thedetector signal across the detector panel, the actual detector responsein ADUs may be determined. For this purpose, an ionization chamber maybe used to measure the exposure at the scanner isocenter (or anylocation within the x-ray FOV) for any arbitrary x-ray techniquesfactors (e.g. kVp, filtration). The ADU values may then be measured fromthe projection image at various locations along the cone and fan angledirections to determine a relationship between the ADU value recorded inthe detector and the exposure at the scanner isocenter. By sweepingthrough all possible tube current values (up to detector saturation), acomplete description of the ADU as a function of exposure at theisocenter may be determined for each combination of x-ray techniquefactors.

FIG. 17A illustrates the measured ADU (near the location of the centralray on the detection plane) as a function of tube current. FIG. 17Billustrates the measured ADU (near the location of the central ray onthe detection plane) as a function of exposure at the isocenter for thebCT scanner. These specific measurements were made using a 60 kV x-raybeam with increasing thicknesses of copper filtration, measured on aprototype bCT system. The various curves illustrate differences in x-raytube (e.g. x-ray source) output related to the amount of added copperfiltration.

A tungsten anode spectral model may be used to compute the polyenergeticx-ray spectrum for a selected kV/filter combination. Monotonicallyincreasing thickness of a breast tissue of any given composition maythen be used to mathematically filter the computed x-ray spectrum.Making use of a photon fluence to exposure conversion factor, theexposure as a function of x-ray attenuation through the breast tissuemay then be calculated. Using these data, and the aforementioned datarelating ADU values to exposure measurements, the ADU may be determinedas a function of path length through the breast for a selected kV/filtercombination.

To equalize the ADU values along the cone angle direction of thedetector, the ADU value resulting from the x-ray path traversing throughthe thickest region of the detector (in the cone angle direction) may beused as the normalization value (ADU₀) since this is the location atwhich the maximum amount of incident photons is necessary to achieve anoptimal signal-to-noise ratio while simultaneously keeping the dose aslow as two-view mammography.

Equalization of the ADU value may be accomplished by an algorithm todetermine the thickness of a given filter material (e.g. aluminum,copper, titanium) that is needed to compensate for the decrease in pathlength relative to ADU₀ as a function of position along the cone angledirection of the detector.

FIGS. 18A-18C illustrate the thickness of the wedge-shape filterdesigned based on the data illustrated in FIG. 16 as a function of thematerial of the filter. The x-axis of FIGS. 16 and 18A-18C represent thecone angle in degrees. A cone angle of 0 degrees corresponds to locationof the central ray.

Specifically, FIG. 18A illustrates the computed filter thickness forcopper wedge-shaped filter as a function of cone angle (dashed line)1802 and a linear regression fit (solid line) 1800. FIG. 18B illustratesthe computed filter thickness for titanium wedge-shaped filter as afunction of cone angle (dashed line) 1806 and a linear regression fit(solid line) 1804. The titanium wedge-shaped filter requires a thickeramount of material because of the relatively lower atomic number (Z=22)compared to copper (Z=29). FIG. 18C illustrates the computed filterthickness for aluminum wedge-shaped filter as a function of cone angle(dashed line) 1808 and a linear regression fit (solid line) 1810.Aluminum (Z=13) attenuates the radiation significantly less than copperand titanium for the same reasons noted above.

FIGS. 19A-19C illustrates wedge-shaped filter designs (i.e. thethickness profiles in the z-direction) 1900, 1902, 1904, 1906, 1908 foreach breast class (e.g. V1, V2, V3, V4 and V5). FIG. 19A illustrates thethickness profile in the z-direction of an exemplary aluminumwedge-shaped filter for each breast class V1, V2, V3, V4 and V5. FIG.19B illustrates the thickness profile in the z-direction of an exemplarycopper wedge-shaped filter for each breast class V1, V2, V3, V4 and V5.FIG. 19C illustrates the thickness profile in the z-direction of anexemplary titanium wedge-shaped filter for each breast class V1, V2, V3,V4 and V5. As it may be seen by comparing curves 1900-1908 on each oneof FIG. 19A, FIG. 19B and FIG. 19C, a wedge-shaped filter made ofaluminum has a much larger thickness (e.g. about 7 to 25 times larger)than wedge-shaped filters made of copper or titanium. An ideal filtershould be thin enough so that it fits into the geometrical constraintsof the particular system, but the filter should also be thick enoughthat it may be machined to a desired shape and form. Additional metricsthat are important to consider when designing a modulation filter mayinclude, for example, differences in scattered radiation levels and beamhardening of the x-ray beam as the x-ray beam traverses the modulationfilter.

Design of the Bowtie-Shaped Filter

Using the same framework as the design of the wedge-shaped filter, aperson's breast image may be used to simulate the x-ray path length as afunction of position along the x-axis of the detector panel (i.e.fan-angle direction) to determine the design of the bowtie-shapedfilter. This process may be repeated for any number of cone angles fromthe posterior to anterior limits of the detector panel.

Referring back to FIG. 2B, ray paths in the fan angle direction incidenton the anterior edge of the detector panel 110 are shown. By measuringthe x-ray path length as a function of fan angle along the entiredetector panel 110, a complete description of the signal attenuation maybe determined. This process may be repeated for any number of views.Results of the combined path length profiles for 215 person images (2orthogonal views for each case) are shown in FIG. 20 using imagesacquired on a prototype bCT system. The curve 2000 illustrated in FIG.20 represents the pathway traveled by the x-rays through the breast fromsuperior part to inferior part of the breast in the x-direction. Thebowtie-shaped filter is designed to compensate to the pathwayillustrated in FIG. 20.

The fan angle path length profiles become much narrower as the coneangle increases towards the anterior part of the breast CT images. Thiseffect in the cone angle direction is accounted for in the wedge-shapedfilter design. Therefore, the fan angle path length profiles at a coneangle of 0° are used in the present invention. This process may berepeated for any number of projections within the 360° motion of the bCTgantry. The example shown in FIG. 20 takes the average fan-angle pathlength profile for all person data sets (e.g. 215 person images, 2orthogonal projections for each image) independent of breast size andshape.

The design considerations for the thickness of the wedge-shape filterare discussed next.

FIGS. 21A-21C illustrate the thickness of the bowtie-shaped filterdesigned based on the data illustrated in FIG. 20 as a function of thematerial of the filter. The x-axis of FIGS. 20 and 21A-21C represent thefan angle in degrees.

FIG. 21A illustrates the computed filter thickness for copperbowtie-shaped filter as a function of fan angle (data points) 2100 and aparabolic fit (solid line) 2102. Copper greatly attenuates theradiation. As a result, a thin bowtie-shaped filter may be formed usingcopper. FIG. 21B illustrates the computed filter thickness for thetitanium bowtie-shaped filter as a function of cone angle (data points)2104 and a linear regression fit (solid line) 2106. Titanium attenuatesthe radiation slightly less than copper. Thus, a titanium bowtie-shapedfilter needs to be slightly thicker than a copper bowtie-shaped filterto achieve the same filtering results. FIG. 21C illustrates the computedfilter thickness for aluminum bowtie-shaped filter as a function of coneangle (data points) 2108 and a linear regression fit (solid line) 2110.Aluminum attenuates the radiation significantly less than copper andtitanium as discussed previously.

FIGS. 22A-22C illustrates bowtie-shaped filter designs (i.e. thethickness profiles in the x-direction) 2200, 2202, 2204, 2206 and 2208for each breast class (e.g. V1, V2, V3, V4 and V5, respectively). FIG.22A illustrates the thickness profile in the x-direction of an exemplaryaluminum bowtie-shaped filter for each breast class V1, V2, V3, V4 andV5. FIG. 22B illustrates the thickness profile in the x-direction of anexemplary copper bowtie-shaped filter for each breast class V1, V2, V3,V4 and V5. FIG. 22C illustrates the thickness profile in the x-directionof an exemplary titanium bowtie-shaped filter for each breast class V1,V2, V3, V4 and V5. As it may be seen by comparing curves 2200-2208 oneach one of FIG. 22A, FIG. 22B and FIG. 22C, a bowtie-shaped filter madeof aluminum has a much larger thickness (e.g. about 5 to 15 timeslarger) than bowtie-shaped filters made of copper or titanium.Accordingly, copper and titanium may be a better choice of material forthe bowtie-shaped filter.

Design of Combined Wedge-Shaped and Bowtie-Shaped Filter

The present invention describes a method for designing a combinedwedge-shaped and bowtie-shaped filter for equalizing the signal at thedetector panel in both the cone angle (i.e. vertical) and fan angle(i.e. horizontal) directions of the detector panel. FIGS. 18A-18C and21A-21C illustrate an exemplary one-dimensional design shapes for awedge-shaped filter and a bowtie-shaped filter, respectively. Theseshapes may be fit to polynomial functions (or other mathematicalfunctions) and then used in tandem to compute the overall shape of asingle 3D-beam modulation filter. FIG. 23 is a diagram of combining abowtie-shaped filter 2302 and a wedge-shaped filter 2304 used in tandem.According to various embodiments, the bowtie-shaped filter 2302 may beprovided adjacent to the wedge-shaped filter 2304 to form the combinedbowtie-shaped and wedge-shaped filter 2300 illustrated in FIG. 23.

FIGS. 24A-24C illustrate 3D shape of an aluminum combined bowtie-shapedand wedge-shaped filter. FIG. 24A illustrates a surface plot 2400 of thealuminum combined filter thickness profile in the z-direction (i.e. coneangle direction) and x-direction (i.e. fan angle direction). FIG. 24Billustrates line profile 2402 corresponding at z=0 cm, line profile 2404corresponding at z=1.9 cm, line profile 2406 corresponding at z=3.8 cm.FIG. 24C illustrates line profile 2408 corresponding at y=0 cm, lineprofile 2410 corresponding at y=1.0 cm, line profile 2412 correspondingat y=−2.25 cm.

Once the plurality of wedge-shaped filters and the bowtie-shaped filtersare determined as illustrated in FIGS. 19A-19C and 22A-22C,respectively, the wedge-shaped filter and the bowtie-shaped filtercorresponding to each profile (e.g. V1, V2, V3, V4 and V5) may becombined to form the combined filter for each profile. FIG. 25illustrates the combined filter shape 2500 of titanium 3D-beammodulation filter for the first profile (e.g. V1, the x-small sizebreast) and the combined filter shape 2510 of titanium of titanium3D-beam modulation filter for the fifth profile (e.g. V5, the x-largesize breast). The combined filter shape 2500 of titanium 3D-beammodulation filter for the first profile (e.g. V1, the x-small sizebreast) is obtained by combining the bowtie-shaped filter design 2200for V1 and the wedge-shaped filter design 1900 for V1. The combinedfilter shape 2510 of titanium 3D-beam modulation filter for the fifthprofile (e.g. V5, the x-large size breast) is obtained by combining thebowtie-shaped filter design 2208 for V5 and the wedge-shaped filterdesign 1908 for V5.

The introduction of the combined filter results in a reduction in theradiation dose delivered to the person while not having a negativeimpact on the image quality. For example, using the combined filtershape 2510, the reduction in the radiation dose on the small size breastcorresponding to breast class V1 is about 33%, the reduction in theradiation dose on the medium size breast corresponding to breast classV3 is about 47%, and the reduction in the radiation dose on the largesize breast corresponding to breast class V5 is about 54%.

Exemplary Method

Embodiments provide a plurality of 3D-beam modulation filters for avariety of breast shapes. A best fitting 3D-beam modulation filter maybe selected for a given person based on the actual shape of the person'sbreast. For example, the breast of the person may be examined by atechnician and a corresponding pre-determined breast class (e.g. one ofV1, V2, V3, V4 or V5 discussed above) may be identified. According tovarious embodiments, the breast of the person may be evaluated using alaser evaluating system to determine a pre-determined breast class thatbest fits the person's breast.

When the person is associated with one of the profiles, a breastimmobilizer associated with the person's identified profile may be usedduring the imaging process to ensure that the breast is centered in thefield of view of the scanner. A 3D-beam modulation filter correspondingto the identified profile may be used for generating CT images ofperson's breast. The use of the combined breast immobilizer and 3D-beammodulation filter corresponding to the specific shape of the person'sbreast enables reduction of the radiation dose on the person withoutcompromising the image quality.

FIG. 26 illustrates a flowchart 2600 of steps for generating a CT imageof a body part using a size or shape specific 3D-beam modulation filter.At step 2602, an actual shape or size of a body part of interest (e.g.breast of a person) may be determined. According to various embodiments,the actual shape or size of the body part may be determined by visualinspection of a person (e.g. x-ray technician) or using a laser-basedevaluation system. In some embodiments, determining the shape or size ofthe body part may include identifying a predetermined profile based onthe shape or size of the body part (step 2604). That is, determining theshape or size of the body part may include determining if the body partis similar in shape and/or size to one of the plurality of predeterminedprofiles.

At step 2606, an immobilizer (e.g. an immobilizing mold or a confiningmask) may be selected among a plurality of immobilizers based on thedetermined shape or size of the body part or the identifiedpredetermined profile. The selected immobilizer may conform to the bodypart to be imaged (e.g. the breast) and may place the body part in thefield of view of the scanner system. In some embodiments, an immobilizermay be formed for each one of the plurality of predetermined profiles.

At step 2608, a 3D-beam modulation filter may be selected among aplurality of 3D-beam modulation filters based on the determined shape orsize of the body part or the identified predetermined profile. Theselected 3D-beam modulation filter may reduce the radiation dosedelivered to the body part without compromising on the image quality. Insome embodiments, a 3D-beam modulation filter may be formed for each oneof the plurality of predetermined profiles.

In some embodiments, the 3D-beam modulation filter may include acombined bowtie-shaped and wedge-shaped filter. Accordingly, abowtie-shaped filter may be selected among a plurality of bowtie-shapedfilters based on the shape or size of the body part or the identifiedpredetermined profile. Similarly, a wedge-shaped filter may be selectedamong a plurality of wedge-shaped filters based on the shape or size ofthe body part or the identified predetermined profile. The selectedbowtie-shaped filter and the selected wedge-shaped filter may becombined into a combined filter.

At step 2610, the 3D-beam modulation filter may be placed at apredetermined distance from an x-ray source of the body scanner systemsuch that the 3D-beam modulation filter is positioned between the x-raysource and the body part being imaged.

At step 2612, the selected immobilizer may be attached to the scannersystem. For example, the immobilizer may be attached to a tabletop ofthe scanner system using one or more of attachment elements. A first endof the attachment element may be attached to the table top and a secondend, opposite from the first end, of the attachment element may beattached to the immobilizer. The body part to be imaged may be placed inthe selected immobilizer (step 2614).

If necessary, the position of the 3D-beam modulation filter may beadjusted to account for variation in the positioning of the body part inthe x-ray field of view of the scanner system. Assessment of thepositioning of the body part and the 3D-beam modulation filter could beaccomplished by acquiring two orthogonal (i.e. 90 degrees separated)“scout view” projection images at a low dose level prior to the bCTimage acquisition (step 2616). These scout views could then be used toeither adjust the position of the 3D-beam modulation filter using amotorized positioning system (e.g. filter positioning system 316illustrated in FIG. 3) and/or the body part could be adjusted in the FOVby the x-ray technician or by the person themselves (step 2618).

At 2620, a CT image of the body part may be generated using the 3D-beammodulation filter to reduce a radiation dose of the body part.

The various participants and elements shown in FIGS. 1-26 may operateone or more computer apparatuses (e.g., a server computer) to facilitatethe functions described herein. Any of the elements in FIGS. 1-26 mayuse any suitable number of subsystems to facilitate the functionsdescribed herein. Examples of such subsystems or components are shown inFIG. 27. The subsystems shown in FIG. 27 are interconnected via a systembus 2800. The subsystems such as a printer 2808, keyboard 2814, fixeddisk 2816 (or other memory comprising computer-readable media), monitor2820, which is coupled to a display adapter 2810, and others are shown.Peripherals and input/output (I/O) devices, which couple to I/Ocontroller 2802, may be connected to the computer system by any numberof means known in the art, such as serial port 2812. For example, serialport 2812 or external interface 2818 may be used to connect the computerapparatus to a wide area network such as the Internet, a mouse inputdevice, or a scanner. The interconnection via system bus allows thecentral processor 2806 to communicate with each subsystem and to controlthe execution of instructions from system memory 2804 or the fixed disk2816, as well as the exchange of information between subsystems. Thesystem memory 2804 and/or the fixed disk 2816 may embodycomputer-readable medium.

Specific details regarding some of the above-described aspects areprovided below. The specific details of the specific aspects may becombined in any suitable manner without departing from the spirit andscope of embodiments of the invention.

Storage media and computer readable media for containing code, orportions of code, may include any appropriate media known or used in theart, including storage media and communication media, such as but notlimited to volatile and non-volatile, removable and non-removable mediaimplemented in any method or technology for storage and/or transmissionof information such as computer readable instructions, data structures,program modules, or other data, including RAM, ROM, EEPROM, flash memoryor other memory technology, CD-ROM, digital versatile disk (DVD) orother optical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, data signals, datatransmissions, or any other medium which may be used to store ortransmit the desired information and which may be accessed by thecomputer. Based on the disclosure and teachings provided herein, aperson of ordinary skill in the art may appreciate other ways and/ormethods to implement the various embodiments.

It may be understood that the present invention as described above maybe implemented in the form of control logic using computer software in amodular or integrated manner. Based on the disclosure and teachingsprovided herein, a person of ordinary skill in the art may know andappreciate other ways and/or methods to implement the present inventionusing hardware and a combination of hardware and software.

Any of the software components or functions described in thisapplication, may be implemented as software code to be executed by aprocessor using any suitable computer language such as, for example,Java, C++ or Perl using, for example, conventional or object-orientedtechniques. The software code may be stored as a series of instructions,or commands on a computer readable medium, such as a random accessmemory (RAM), a read only memory (ROM), a magnetic medium such as ahard-drive or a floppy disk, or an optical medium such as a CD-ROM. Anysuch computer readable medium may reside on or within a singlecomputational apparatus, and may be present on or within differentcomputational apparatuses within a system or network.

The above description is illustrative and is not restrictive. Manyvariations of the invention may become apparent to those skilled in theart upon review of the disclosure. The scope of the invention may,therefore, be determined not with reference to the above description,but instead may be determined with reference to the pending claims alongwith their full scope or equivalents.

One or more features from any embodiment may be combined with one ormore features of any other embodiment without departing from the scopeof the invention.

A recitation of “a”, “an” or “the” is intended to mean “one or more”unless specifically indicated to the contrary.

1.-20. (canceled)
 21. A method for producing three-dimensional (3D) beammodulation filters for performing cone beam computed tomography (CBCT)breast imaging, comprising: receiving a plurality of CBCT datasetsacquired using a CBCT system from a plurality of persons with differentbreast shapes and sizes; selecting a subset of the CBCT datasets todefine a breast class, said breast class having a breast class x-raypath length profile; fabricating, using the breast class x-ray pathlength profile, a three-dimensional (3D) beam modulation filter to havea thickness profile that compensates for intensity variations of ameasured signal at a detector when a person's breast within said breastclass is imaged by a cone beam of the CBCT system.
 22. The method ofclaim 21, wherein the cone beam comprises a first beam angle directionand a second beam angle direction, wherein the first beam angledirection is perpendicular to the second beam angle direction, whereinthe thickness profile varies as a function of position in both the firstbeam angle direction and the second beam angle direction.
 23. The methodof claim 22, wherein the first beam angle direction is a fan angledirection and the second beam angle direction is a cone angle direction.24. The method of claim 22, wherein the thickness profile is calculatedfrom the breast class x-ray path length profile to compensate forintensity variations of the measured signal at the detector resultingfrom differences in path length as a function of position in the firstand second beam angle directions.
 25. The method of claim 24, whereinthe thickness profile is calculated for a filtration material, thefiltration material being at least one of aluminum, copper, andtitanium.
 26. The method of claim 21, wherein the cone beam comprises afirst beam angle direction, wherein the 3D beam modulation filtercomprises a first portion with a first thickness profile that varies asa function of position in the first beam angle direction, wherein thefirst thickness profile is calculated from the breast class x-ray pathlength profile to compensate for intensity variations of the measuredsignal at the detector resulting from differences in path length as afunction of position in the first beam angle direction.
 27. The methodof claim 26, wherein the cone beam further comprises a second beam angledirection, wherein the first beam angle direction is perpendicular tothe second beam angle direction, wherein the 3D beam modulation filtercomprises a second portion with a second thickness profile that variesas a function of position in the second beam angle direction, whereinthe second thickness profile is calculated from the breast class x-raypath length profile to compensate for intensity variations of themeasured signal at the detector resulting from differences in pathlength as a function of position in the second beam angle direction. 28.The method of claim 27, wherein the first thickness profile iscalculated for a first filtration material, wherein the second thicknessprofile is calculated for a second filtration material that is differentfrom the first filtration material.
 29. The method of claim 21, whereinthe breast class is a first breast class, the method further comprising:selecting a second subset of the CBCT datasets to define a second breastclass, said second breast class having a second breast class x-ray pathlength profile; fabricating, using the second breast class x-ray pathlength profile, a second 3D beam modulation filter to have a secondthickness profile that compensates for intensity variations of ameasured signal at a detector when a second person's breast within saidsecond breast class is imaged by the cone beam of said CBCT system. 30.The method of claim 21 further comprising selecting a plurality ofsubsets of the plurality of CBCT datasets to define a plurality ofbreast classes, each breast class having a different breast class x-raypath length profile; fabricating, using the different breast class x-raypath length profiles, a plurality of 3D beam modulation filters to eachhave a different thickness profile that compensates for intensityvariations of a respective measured signal at a detector when a person'sbreast within each respective breast class is imaged by the cone beam ofsaid CBCT system, wherein the plurality of subsets are fewer than theplurality of CBCT datasets.
 31. The method of claim 30, wherein theplurality of CBCT datasets comprise at least 200 CBCT datasets, whereinthe plurality of subsets comprise at most 10 subsets.
 32. The method ofclaim 29, wherein each subset is selected based on at least one of aplurality of characteristics, said characteristics comprising a breastdiameter at chest wall, a breast volume, and a breast length.
 33. Themethod of claim 29, wherein each subset is selected based on aclassification of each CBCT dataset into one of a plurality of volumepercentile groups.
 34. The method of claim 29, wherein each subset isselected based on a classification of each CBCT dataset into one of aplurality of breast sizes.
 35. The method of claim 29, wherein eachsubset is selected based on a classification of each CBCT dataset intoone of a plurality of breast shapes.
 36. The method of claim 22, whereinthe thickness profile is calculated from an average radius profile ofthe breast class, wherein the average radius profile is an average overa plurality of measured radius profiles for each CBCT dataset in theselected subset.
 37. The method of claim 36 further comprisingfabricating, using the average radius profile of the breast class, abreast phantom for the breast class.
 38. The method of claim 37 furthercomprising fabricating an immobilizer for the breast class using thebreast phantom as a mold.
 39. A three-dimensional (3D) beam modulationfilter for performing computed tomography (CT) imaging of a person'sbreast, produced by the method of claim
 21. 40. A three-dimensional (3D)beam modulation filter for performing cone beam computed tomography(CBCT) imaging of a bodypart of a person, wherein said bodypart, whenimaged by a CBCT system, has an x-ray path thickness profile that varieswith a cone angle and varies with a fan angle of a cone beam used toimage said bodypart, said 3D beam modulation filter comprising an x-raypath thickness profile that varies with said cone angle and varies withsaid fan angle of said cone beam used to image said bodypart so as tocounter intensity variations of a measured signal at a detectorresulting from said x-ray path thickness profile of said bodypart whensaid bodypart is imaged by a cone beam of the CBCT system.